Turning operation on lathe and study of effect of speed, feed and depth of cut on surface roughness

Machining Turning Definition: A machining operation in which a single point cutting tool removes material from a rotating workpiece to form a cylindrical shape. 2

Basic Mechanics of Metal Cutting Metal ahead of the cutting tool is compressed. This results in the deformation or elongation of the crystal structure -resulting in a shearing of the metal. As the process continues, the metal above the cutting edge is forced along the “chip-tool” interference zone and is moved away form the work.

Direction of feed motion decides type of turning – Feed motion parallel to axis – longitudinal turning Feed motion perpendicular to axis – transverse turning Source: NPTEL

Machining Orthogonal cutting model Cutting edge is perpendicular to the cutting speed 5

Machining Terminology Speed – surface cutting speed (v, m/min) Feed – advancement of tool through the workpiece in one rotation of spindle (f, mm/rev) Depth of cut – distance by which tool penetrates in the workpiece (d, mm)

Process Parameters Any machining operation involves three basic parameters Primary motion (cutting motion or Cutting Speed) v During the rotation of jobpiece, primary motion makes workpiece to approach cutting edge hence spindle rotates anticlockwise Secondary motion (feed) f Distance travel along the axis Depth of cut d Distance tool penetrates in the jobpiece. It is along raidial directoion Relative tool travel in 1 rotation = pD Cutting Speed = v = pDN/1000 m/min

Process Parameters Three process parameters are perpendicular to each other

Output Parameter: Surface Roughness Roughness: It is defined as closely spaced, irregular deviations on a scale smaller than that of waviness. Roughness is expressed in terms of its height, its width, and its distance on the surface along which it is measured. Waviness: It is a recurrent deviation from a flat surface, much like waves on the surface of water. It is measured and described in terms of the space between adjacent crests of the waves (waviness width) and height between the crests and valleys of the waves (waviness height).

Sample Plot: Surface Roughness v/s Cutting Speed

Cutting Speed Depends on number of rotations (rpm) of the chuck of a lathe Always expressed in meters per minute (m/min) Important to use correct speed for a given material to get desired surface finish Too high: cutting-tool breaks down rapidly Too low: time lost, low production rates

Cutting Speeds Using High-Speed Steel Toolbit Turning Rough Cut Finish Cut Material m/min Machine Steel 27 30 Tool Steel 21 Cast Iron 18 24 Bronze Aluminum 61 93

Calculating Spindle Speed Given in revolutions per minute Cutting speed of metal and diameter of work must be known Proper spindle speed set by dividing CS (m/min) by circumference of work (mm)

Example: Calculate RPM required to rough-turn 50 mm diameter piece of machine steel (CS 27 MPM):

Effect of Cutting Speed Rough Finish v (m/min) 18.5 37.1 51.8 69.1 86.4 f (mm/rev) 0.3 d (mm) As speed increases surface finish increases

Feed Distance cutting tool advances along length of workpiece for every revolution of the spindle Feed of lathe dependent on speed of lead screw or feed rod Speed of feed rod controlled by change gears in quick-change gearbox

Feed Lead Screw Feed Rod

Feed f – the distance the tool advances for every rotation of workpiece (mm/rev)

Feed Analysis of Turning Feed & Feed rate: Both are used to specify the same quantity which is the amount of travel in secondary motion direction in a specified amount of time. Time unit is revolution of workpiece for feed while feed rate is defined in terms of second. 19

Effect of Feed on Roughness

Rough and Finish cut Generally two cuts are employed - Roughing cut Purpose to remove excess material quickly Coarse feed: surface finish not too important 0.25- to 0.4-mm Finishing cut Used to bring diameter to size Fine feed: Produce good finish 0.07- to 0.012-mm

Effect of Feed v (m/min) 25.9 f (mm/rev) 0.5 0.3 0.1 d (mm) Finish Rough v (m/min) 25.9 f (mm/rev) 0.5 0.3 0.1 d (mm) At low feed surface finish is good.

Feed High-speed steel cutting tool feed per revolution of spindle Rough Cuts Finish Cuts Material mm/rev Machine Steel 0.25-0.50 0.07-0.25 Tool Steel Cast Iron 0.40-0.65 0.13-0.30 Bronze Aluminum 0.40-0.75 0.13-0.25

Depth of Cut Distance tool penetrates in the workpiece It is equal to half the difference between the initial diameter and the final diameter. Generally only one roughing and one finishing cut is used Roughing cut should be as deep as possible to reduce diameter to within 0.75 to 1 mm of size required Finishing cut should be less than 0.125 mm

Depth of cut on a lathe

Depth of Cut perpendicular distance between machined surface and uncut surface of the Workpiece d = (D1 – D2)/2 (mm)

Effect of Depth of Cut v (m/min) 25.9 f (mm/rev) 0.3 d (mm) 0.5 0.1 At low depth of cut surface finish is good.

Material Removal Rate MRR Volume of material removed in one revolution MRR =  D d f mm3 Job makes N revolutions/min MRR =  D d f N (mm3/min) In terms of v MRR is given by MRR = 1000 v d f (mm3/min)

MRR Note that undeformed chip thickness is equal to feed! 29

Calculating Machining Time Factors such as spindle speed, feed and depth of cut must be considered

Example: Calculate the time required to machine a 2-in. diameter machine-steel shaft 16 inch to 1.850 diameter finish size Roughing cut Roughing feed feed = .020 Total Time 29.1 min Finishing cut: Finishing feed = .003

For animations on effect of Depth of Cut, click the following Low Depth of Cut Medium depth of cut High depth of cut

Effect of Depth of Cut Cutting Force vs. Depth of cut for a mild steel shaft at 118 RPM and feed rate 0.3 mm/rev

Effect of Feed Rate Cutting Force Vs. Feed Rate for a mild steel shaft at 118 RPM with a depth of cut of 0.5 mm

Effect of Cutting Speed Cutting force vs. Cutting speed at different feed rates for a mild steel shaft cut with depth of cut of 0.3 mm